Peel and cut the potatoes in medium sized pieces and add it to the pressure cooker.
Wash the dal (toor dal - yellow pigeon peas) and add it to the pressure cooker.
Add the green chili and shallots to the cooker.
Add all the spices, salt and water, mix everything well (if the potatoes you used are large you may need more water, adjust accordingly).
Close the lid of the pressure cooker with the safety valve.
Cook in medium heat for about 10 to 12 minutes (after the steam begins to release) or until 3 whistles come.
After 12 minutes of cooking, switch off the pressure cooker and allow it to cool down.
Meanwhile you can prepare the tamarind by dissolving the tamarind paste in about ½ cup of water. You can keep the tamarind in water for about 5 to 10 minutes so it will dissolve nicely.If the tamarind you use has strings (fibers), filter this tamarind after its dissolved and the tamarind mixture is ready to be use. Keep this aside. (If it's store bought and there are no strings, you can use it directly).Instead of using tamarind which needs to be dissolved in water and filtered, you can also use tamarind extract which can be directly added (I have given the link).
Once the pressure cooker is cooled, you can open the lid and see if the potatoes and dal are cooked and soft.
Add the tamarind and also add more water if needed, and bring the mixture to a boil. Heat for another 1 minute and the potato sambar is ready, you just need to prepare the seasoning for the sambar if you wish to add the seasoning. You can add even more water if you prefer less thickness for the sambar, as potato sambar thickens pretty fast. As it cools down, potato sambar thickens even more more but you can always add more water as needed and heat the sambar before serving. To add seasoning (if using)Heat oil in a pan and add mustard seeds.
When the mustard seeds splutter, add broken dry red chilies and curry leaves and lightly fry. Make sure there is no water content in the leaves.
Pour this prepared seasoning over to the sambar and gently mix. You may also use a light seasoning just with mustard seeds or curry leaves. (It's good to have all ingredients for seasoning, but this is not a must).
Garnish with chopped cilantro leaves if you like! (Again, this is optional. It is always good to garnish sambar with cilantro leaves as it adds more flavor to the sambar, but its not a must for this potato sambar)